1. Field of Invention
The invention relates to parts feeders, and more particularly to feeders capable of analyzing small, electronic, mechanical or food parts or other objects in a selection zone and, if required, reorienting the parts to a desired orientation.
2. Description of Related Art
To automate the assembly of mechanical components, parts must be precisely oriented prior to packing or insertion. The manufacturing industry currently relies on passive parts feeders using handcrafted mechanical "filters" that admit only those parts having a desired orientation. Rejected parts are recycled for another pass through the orientation mechanism. When part geometry changes, the "filter" must be mechanically redesigned, involving a trial-and-error process typically requiring several months and up to 50% of the costs of the automated assembly cell.
Currently, the most common method for orienting parts is a vibratory bowl feeder, in which parts in a specially configured bowl are vibrated with a rotary motion so that they climb a helical track. As they climb, a sequence of baffles and cut-outs in the track creates a mechanical "filter" that causes parts in all but one orientation to fall back into the bowl for another attempt at achieving a desired orientation. It is also possible to design the track to mechanically rotate parts into a desired orientation. Other orientation methods use centrifugal forces, belts, or reciprocating forks, rather than vibration, to move parts through the part-feeding mechanism.
When part geometry changes in a bowl feeder, specialists must redesign the bowl and/or other components of the mechanism to accommodate the new parts. The mechanism must be tested and modified numerous times to achieve the best feeding efficiency and to eliminate jams. Jams are, for example, caused by the fact that reorientation and "filtering out" of parts by means of baffles and cut-outs in the track most often rely on external, loose tolerances or uncontrolled part features.
Another prior art feeder uses an array of nests (silhouette traps) cut into a vibrating plate. The plate and nests vibrate so that parts will remain in the nests only in a particular orientation. By tilting the plate and letting parts flow across it, the nests eventually fill up with parts in the desired orientation. Although the vibratory motion is under software control, specialized mechanical nests must still be designed for each new type of part, and jamming and improper part alignment in the nests are problems, as with bowl feeders. Several other designs for programmable parts feeders have been proposed, in which programmed vibration is used to drive parts into a stable orientation. These methods are useful for bringing parts into "low-energy" positions and orientations, where their respective centers of mass are as low as possible, but other methods are then required to further orient the parts relative to a selection plane.
Sensors, such as tactile probes, photocells, fiber optic sensors and vision systems, have been used to determine the position and orientation of parts delivered by a vibratory track. Once part position and orientation are determined, air jets and trap doors are used to group parts in similar positions and orientations. Difficult and time-consuming physical changes must be made in the components of such systems when part type changeover is desired.
For decades, researchers have studied the "bin picking" problem, that is, the problem of picking a part out of a bin of jumbled parts. Due to the difficulty of recognizing overlapping parts in arbitrary orientations, few of these systems have been adopted for industrial applications. Standard vision systems, however, are often successful when additional constraints are imposed, such as presenting parts in isolation on a flat surface. U.S. Pat. No. 4,876,728 to Roth, which is incorporated herein by reference, discloses an improved vision system for distinguishing touching parts on a conveyor. The vision system processes binary images and recognizes objects based on boundary features, such as lines, arcs, corners and holes, instead of "blob features." The system is interfaced to a robot system and can recognize up to five parts per second. The ability of the vision system to quickly and reliably recognize parts is still dependent, though, on the orientation and degree of overlap of the parts being inspected.
Unlike bowl feeders, which present parts in their final desired orientation, parts presented on flat conveyor surfaces, such as shown in Roth, may lie in one of several stable states. Typically, these stable states are not the desired final orientation for the parts. In general, the parts may require translation and rotation through six degrees of freedom in order to reach their final orientation and destination. While it is possible to use a conventional six axis robot to acquire parts from the flat conveying surface, in general, such robots are not as cost effective, fast, or precise as robots having less degrees of freedom. These drawbacks of the six axis robot decrease the overall effectiveness of the parts feeder.
In addition, six axis robots generally have their three axes of rotation of the wrist intersecting at a single point. This results in a substantial distance between these axes of rotation and the tip of the robot fingers due to the volume required by the wrist and the offset for the fingers. This wrist and finger arrangement has the following drawbacks. First, use of this arrangement will require the designation of a substantial area for the part selection zone so that the wrist and finger arrangement can be properly manipulated to pick up a part. This leads to an overly large-construction of the parts feeder. Second, because some applications will require, during part pick-up, that the longitudinal axes of the fingers be inclined relative to the conveyor surface in order to properly reposition the part, the ability of the gripper to select a part from among a plurality of parts crowded together may be significantly impaired. Third, because the above wrist and finger arrangement has at least two of the three axes of rotation displaced substantially from the center of the picked part, repositioning of the part to its drop or assembly location may require the fingers to assume a position which impedes or prevents final positioning or assembly.
Parts feeders; are known that convey parts through a selection zone, where parts are analyzed by a machine vision system and properly oriented parts are removed by a robot. An example of such a parts feeder developed by the assignee of the present invention is disclosed in U.S. application Ser. No. 08/428,679, filed Apr. 25, 1995, which is incorporated herein by reference. As shown in the aforementioned application, parts are presented at random orientations and positions on a conveyor to the vision system. Non-selected parts are recirculated through the selection zone after being vibrated or otherwise induced to assume new orientations, until the parts are selected.
When parts are scattered onto the selection zone, they end up in various stable states. In order to achieve the desired throughput, the vision system is programmed to identify stable states with a high probability. In general, those high probability stable states are not identical with the desired assembly orientation. As a result, a secondary operation is often needed to transform the part to the desired assembly or place of orientation. This secondary transformation operation is typically performed by the robot itself and/or the part is placed on an external flip station and subsequently picked up again by the robot. As a result, the robot system design becomes more complex, an external part-specific flip station may be needed, and the robot cycle time and overall system cost increases. Further, because such vibration, etc. provides random, uncontrollable orientation of parts, the parts may be recirculated through the selection zone many times before achieving the desired orientation. Such recirculation may cause undesirable wear on the parts.
There is, accordingly, a need for a flexible, general-purpose compact parts feeder that can handle a large variety of parts without requiring mechanical adjustments or physical alterations to the parts feeder and which presents parts in the desired (final or close to final) assembly or place orientation to avoid or minimize costly subsequent part manipulations performed by the robot, its gripper or external devices. There is also a need for a parts feeder which reduces or eliminates undeterministic and uncontrollable recirculation to avoid parts wear. There is a further need for a parts feeder that reduces cycle time fluctuation.
There is also a need for a parts feeder that can automatically find or optimize its own part-feeding control strategy, and whose throughput can be predicted and whose control and design parameters can be derived from a computer-aided design model of the part by means of computer simulation and the derived parameters can then be downloaded to the feeder in order to expedite programming and deployment. Similarly, the same simulation techniques can be used to suggest part design changes at an early design stage in order to improve parts feeding behavior.
There is a further need for a parts feeder that avoids jamming problems associated with sliding movement of parts along surfaces, and that can convey parts at a variable speed to present a larger or smaller number of parts, as needed. There is also a need for a simple and easily constructed parts feeder that is compact in size, cost-effective, efficient and readily installed in existing assembly and material handling work cells.